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CONTENTS 1. About DCSteg 2. Building 3. Using 4. The algorithm of watermarking 5. License ################################################## 1. About DCSteg ################################################## DCSteg is steganographic/watermarking utility that can imprint ASCII strings into image files (jpg, png). Ouput format is png. The approach is to embed the message data in the frequency domain. Some measures such as choosing low frequencies and using error-correcting codes make the message somewhat resistant to minor local image modifications (blur, drawing into the image, etc.). Resizing/cropping the image makes the message illegible, though. There are three possible "strength of imprints" to choose from (0-2). The higher the number, the more durable and visible the imprint is (2 is visible with bare eye, but for someone not looking for hidden text could be attributed to JPEG artifacts). There is limitation for the message to be only ASCII characters (program could be fairly easily modified to support Unicode characters, though). See the algorithm section for further details. WARNING: this is just a toy program, careful statistical analysis of the histogram would probably reveal that there is a hidden image. See http://niels.xtdnet.nl/stego/ and http://www.citi.umich.edu/u/provos/papers/defending.ps for details on steganalysis. Use of PRNG to choose the blocks and order of imprint should make it more resilient to steganalysis (PRNG would use key/passphrase to derive seed). WARNING: in some countries this program might be illegal. ################################################## 2. Building ################################################## In the DCSteg directory, run 'ant'. It builds the class files and the DCSteg.jar file. Run 'ant javadoc' to get javadoc. Alternatively you could import the code into IDE like Eclipse and build it there. The code has no dependencies except the standard JDK version >= 1.4, also works under 6.0. ################################################## 3. Using ################################################## Run 'java -jar DCSteg.jar' to see the usage. Examples of usage: Analysis example (run this first): java -jar DCSteg.jar a input_image.jpg The output of analysis might look like (numbers 0-2 on the left say how many bytes are available for that given imprint strength): Analyzing image, it may take a while if the image is large 0: 3952 bits, after encoding 278 bytes available for the message 1: 3616 bits, after encoding 254 bytes available for the message 2: 2128 bits, after encoding 148 bytes available for the message Encoding example: java -jar DCSteg.jar e input_image.jpg 0 output_image.png "This is the message" Decoding example: java -jar DCSteg.jar d imprinted_image.jpg 0 ################################################## 4. The algorithm of watermarking ################################################## First, image is converted from RGB colorspace to YCbCr colorspace. Then it is split in blocks of 8x8 pixels, each block is transformed using DCT into frequency domain and quantized (note the similarity to JPEG). DC coefficients are used to carry the message bits. A coefficient must be large enough to hold the message bit, that means at least one higher bit has to be set than the imprint bit order (imprint strength). Embedding iterates top to bottom, left to right. Thus if the image is modified so that a block capable of holding message bit is created/removed, rest of the bits in the message are shifted and thus illegible for human. Before the message itself is embedded, it is encoded using the Hamming(7,4) error-correcting code with 7-bit codewords encoding 4 bits of message. Each code word can correct single-bit error (multiple-bit errors in a single codeword cannot be detected). Decoding is just the encoding process in reverse, except the Hamming code is used to correct errors if found. ################################################## 5. License ################################################## License: GPL v2 (see http://www.gnu.org/licenses/old-licenses/gpl-2.0.html) Author: Ondrej Mikle (ondrej.mikle \at gmail.com)
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2005-ish code for steganographic hiding of messages using discrete cosine transform and Hamming self-correction
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